The present invention relates to improvements in a tapper incorporating a reverse rotation mechanism in which the direction of rotation can be switched smoothly all the time, from positive to reverse or reverse to positive, so as to prevent accidental damage to the tap.
As disclosed in U.S. Pat. No. 3,041,893, tappers of this type incorporating a reverse rotation mechanism generally comprise a tapper main body to be connected to the drive portion of machine tool and rotatable in a specified direction, a positive rotation clutch rotatable with the main body and a reverse rotation clutch rotatble in the opposite direction to the main body. The driven shaft of the tapper is provided with a clutch pawl which is selectively engageable in grooves in the clutches to rotate the driven shaft in positive direction or reverse direction in changeable manner.
The clutch pawl employed in the conventional construction is usually in the form of a cylindrical pin and is provided integrally with the driven shaft. Accordingly, when the pin disengages from the groove of the positive rotation clutch to switch positive rotation to reverse rotation, the edge of grooved lower end of the clutch tends to engage with or bear against the pin, depressing the pin with the resulting reaction and thus exerting an impact load on the pin in the direction of thrust. The impact load is further delivered to the driven shaft immediately, with the result that the excess load frequently causes damage to the ridge of screw thread in the workpiece or to the tap. The same situation is also experienced when the pin comes into or is brought out of engagement with the groove of the reverse rotation clutch. At this time, the driven shaft is subjected to an upward thrust load, which similarly entails damage to the ridge of screw-thread or to the tap.
In an attempt to prevent the edge of the grooved portion of the clutch from engaging the clutch pawl and to thereby avoid occurrence of thrust impact load, a square or rectangular pawl has been proposed recently. Although the clutch pawl is effectively operable without involving the above-mentioned objectionable phenomenon and therefore free of thrust impact load, such improved result is available only for a very short early period of use. Through repeated use, in fact, the corners of the clutch pawl and of the grooved portion of the clutch are inevitably worn away and become rounded, with the result that the driven shaft is subjected to the above-mentioned thrust impact load which causes damage to the ridge of screw-thread in the workpiece or to the tap. Furthermore with the conventional construction in which the clutch pawl is made integral with the driven shaft, the aforesaid wear of the parts, once initiated, gives rise to a thrust impact repeatedly which in turn produces further wear on the parts. Thus the drawback described is all the more pronounced to result in the serious defect that the device becomes inoperable in a short period of use.